1. Field of the Invention
The present invention relates to a printed wiring board to which electronic parts are connected by soldering.
2. Description of the Background Art
FIG. 9 is a plan view of a conventional printed wiring board B5, looking from the printed wiring side. In the figure, the printed wiring board B5 comprises a substrate 4, an insertion opening 2 into which the lead of an electronic part is inserted, a land 6 serving as a soldering foundation, and a solder resist 3 that avoids soldering in other than the land 6. For the land 6, copper foil or the like is employed which is the printed wiring material. For the solder resist 3, an insulating synthetic resin is employed which has the property of preventing adhesion of solder.
The land 6 of FIG. 9 is in an oval-shape. The advantage of this resides in that a plurality of the lands 6 aligned in direction P provide greater land area than obtainable in employing a circular land of a diameter equal to the short diameter of the land 6 (the maximum diameter in direction P), thereby increasing the amount of solder adherent thereto.
Unfortunately, solder bridge, which is the phenomenon that solder bridges two or more lands, is liable to occur because length L2 of the proximal peripheries of the adjacent lands 6 separated distance L1 apart is long as shown in FIG. 9.
FIG. 10 shows a printed wiring board B6 having a rhombic land 7 which incorporates an improvement in preventing solder bridge. The printed wiring board B6 has the same construction as the printed wiring board B5, except that the land 6 is replaced with the rhombic land 7 in which the length of diagonals correspond to the long diameter (the maximum diameter in a direction orthogonal to direction P) and the small diameter of the land 6, respectively. The diagonal having the length of the small diameter of the land 6 is aligned in direction P. Thereby, as shown in FIG. 10, length L2 of the proximal peripheries of the adjacent lands 7 can be reduced considerably, so that the possibility of forming solder bridge is quite remote.
The rhombic land 7 is, however, unsuitable in terms of soldering, because the area of the rhombic land 7 is smaller than that of the oval-shaped land 6. Specifically, when soldering is performed by, for example, immersing the printed wiring board B6 into a solder bath storing a molten solder, the amount of solder adherent thereto is insufficient because the area of the land 7 is small. Less amount of solder may reduce the strength of solder in the event that products mounting the printed wiring board B6 are subjected to impact such as vibration in shipping, and impact due to heat cycle such as temperature change in the environment in use, or temperature change at the turning on/off a power supply.
To avoid the above problem, it is necessary to search for part having less amount of solder adherent thereto and then add solder to the corresponding part. This increases the processing time needed in manufacturing products mounting the printed wiring board B6. In addition, there is the possibility of the secondary fault such as skipping of search in the process to add solder, and solder bridge formation due to drips of the added solder, thus failing to ensure a high degree of reliability of soldering.
As the shape to minimize length L of the proximal peripheries of the adjacent lands spaced distance L1 apart, a convex polygon including vertexes other than the respective vertexes of a rhombus is considered. Such a convex polygon can increase the amount of solder adherent thereto because its area is greater than that of a rhombus. However, its shape approaches the oval-shape shown in FIG. 9, and thus the possibility of solder bridge formation is higher than that of the rhombic land. The possibility of solder bridge formation is high particularly when soldering is made by bringing a plurality of lands into contact with a molten solder at one time.
According to a first aspect of the present invention, a printed wiring board comprises: a substrate having a surface; an insertion opening being provided in the substrate and opening into the surface such that a part""s lead passes through the substrate; and a conductive land surrounding the insertion opening on the surface and having a periphery of a shape which is obtained by providing a projection on at least one side of a rhombus.
According to a second aspect, the printed wiring board of the first aspect is characterized in that the projection is provided on each side of the rhombus.
According to a third aspect, the printed wiring board of the first aspect is characterized in that the projection is in the shape of a right triangle, a hypotenuse of which is in contact with the one side of the rhombus, the hypotenuse being shorter than the one side of the rhombus.
According to a fourth aspect, the printed wiring board of the first aspect is characterized in that the projection has a tip and the tip is located within an area of a right triangle defined outside of the rhombus, a hypotenuse of the right triangle corresponding to the one side of the rhombus and two sides other than the hypotenuse being parallel to either of diagonals of the rhombus.
According to a fifth aspect, the printed wiring board of the first aspect is characterized in that the projection has a tip; a plurality of the lands are provided; and a bisector of an angle of the tip of the projection in one the land and another bisector of an angle of said tip of said projection in the other the land adjacent one the land are not disposed coaxially.
According to a sixth aspect, a printed wiring board comprises: a substrate having a surface; an insertion opening being provided in the substrate and opening into the surface such that a part""s lead passes through the substrate; a conductive land surrounding the insertion opening on the surface; and at least either a solder resist or a material for indication, having a periphery of a shape which is obtained by providing a projection on at least one side of a rhombus, and covering the land such as to expose the land.
According to a seventh aspect, a method of manufacturing a printed wiring board comprises the steps of: a first step of preparing a substrate having a conductive land formed on a surface and an insertion opening that opens into the land to allow a part""s lead to pass therethrough; and a second step of forming at least either a solder resist or a material for indication on a surface of the land, and exposing the surface of the land such as to have a periphery of a shape which is obtained by providing a projection on at least one side of a rhombus.
The use of the printed wiring board according to the first aspect enables to minimize the length of the proximal peripheries of the adjacent lands, thus reducing the possibility of solder bridge formation. In addition, the area of the land is greater than that of a rhombic land, permitting to adhere more solder than the case of soldering to the rhombic land. This increases the strength of solder and leads to a high degree of reliability.
The use of the printed wiring board according to the second aspect avoids solder bridge formation to be caused by providing a plurality of projections.
The use of the printed wiring board according to the third aspect ensures that more solder is adhered than the case of soldering to a rhombic land, while the possibility of solder bridge formation is reduced because the projection has a perpendicular tip.
The use of the printed wiring board according to the fourth aspect does not increase the possibility of solder bridge formation than the case of employing a rhombic land because, when a plurality of lands are disposed such that the diagonals of the rhombus lie on the same axis, there is no possibility that the distance between the tip of the projection and an arbitrary part of the land adjacent thereto is smaller than the distance between the proximal peripheries of the adjacent lands.
The use of the printed wiring board according to the fifth aspect enables to further reduce the possibility of solder bridge formation because the tip of a projection and the tip of the adjacent land extend in different directions.
The use of the printed wiring board according to the sixth aspect enables to minimize the length of the proximal peripheries of the adjacent lands, thus reducing the possibility of solder bridge formation. Since the area of the land is greater than that of a rhombic land, it is possible to adhere more solder than the case of soldering the rhombic land, thereby increasing the strength of solder and leads to a high degree of reliability. Also, a large adhesion area of the land to the substrate lessens the possibility that the land comes off the substrate. In addition, it is possible to further reduce the possibility of solder bridge formation by a tall barrier between the adjacent lands when the solder resist and the material for indication are accumulated on the surface of the land.
With the method of manufacturing a printed wiring board according to the seventh aspect, the step of forming a solder resist or a material for indication can serve for the process of exposing the land such as to have the shape in which a projection is provided on each side of a rhombus. This lowers the possibility of solder bridge formation and facilitates to obtain a printed wiring board with high strength of solder.
It is therefore an object of the present invention to provide a printed wiring board with a high degree of reliability which is attained by preventing solder bridge formation between adjacent lands when a molten solder is allowed to adhere to the lands and the leads of electronic parts, and by increasing the strength of solder with an increased amount of solder adherent thereto.
These and other objects, features, aspects and advantages of the present invention will become more apparent from the following detailed description of the present invention when taken in conjunction with the accompanying drawings.